1. Field of the Invention
This invention relates to the purification of liquid metals, and more particularly to apparatus for and method of purification of liquid metals by exposing the metal to vacuum.
2. Description of the Prior Art
Purification of liquid metals, especially liquid steels, by exposing the metal to subatmospheric pressure, commonly called vacuum, has long been known as shown, for example, in U.S. Pat. No. 963,652. Indeed, the benefits of using vacuum to remove dissolved gases and gases formed during chemical reactions of other elements in liquid steel was recognized by some metallurgist in the 19th century, but the first large scale trials of vacuum treatment of liquid steel did not take place until high capacity vacuum pumps became available and steel grades with improved physical properties were demanded by the fast emerging aircraft, electrical and other advanced industries, in the late 1940s.
Recognition of the metallurgical advantages available by vacuum degassing led to the development of vacuum treatment systems including stream degassing, recirculation degassing, and tank degassing systems. While stream degassers are used primarily for removal of hydrogen from liquid steel for heavy forgings, the recirculation and tank degassers are widely used for large scale vacuum treatment of many grades of steel utilized in a wide range of applications.
Recirculation degassers move a portion of the liquid metal from a ladle into a vacuum chamber through a snorkel attached to the vacuum chamber and having its lower end submerged into the liquid metal in the ladle. DH type degassers employ only one snorkel, with the liquid metal alternately flowing upwardly into the chamber for exposure to vacuum, then back into the ladle. DH recirculating degassing systems are disclosed in U.S. Pat. Nos. 2,906,521 and 2,929,704.
In a RH type recirculating degasser, two snorkels are employed. Liquid metal is caused to flow upwardly through one snorkel by the injection of lift gases, then returned through the second snorkel after exposure to vacuum in the treatment chamber. RH degassing systems are disclosed in U.S. Pat. Nos. 2,893,860 and 3,099,699.
Tank degassers were among the first commercial degassers employed to successfully treat molten steel and many of these devices are also in operation in steel plants around the world. Known tank degassers are shown in U.S. Pat. Nos. 1,131,488 and 2,993,780. In the typical tank degasser, a container or ladle holding a quantity of liquid steel, usually with some slag on top, is placed on a ladle stand in an open top vacuum tank by an overhead crane or the like. A gas or electromagnetic stirring system is then actuated, and the tank cover is placed on the tank, normally by use of a tank car which lowers the cover directly onto the tank. A vacuum-tight closure is affected between the top and tank, normally by an O-ring seal positioned between flanges located one on the tank top and the other on the cover. Usually, the tank is connected to the vacuum pumps by a large diameter off gas line. The stirring operation exposes an increased surface area of liquid steel to the vacuum.
In the typical tank degassing operation, gas stirring is used in which an inert gas, typically argon, is injected into the bottom of the ladle through gas permeable refractory elements. Before, during and after degassing, the temperature and chemical composition of the metal are monitored and alloy materials may be added, as required.
In a variation of the tank degasser just described, the tank and ladle are supported for movement along a track to a position beneath the cover, after which the cover may be lowered or the tank raised to affect the seal. In either embodiment, the off gas line may be connected either to the tank cover or to the tank.
It is also known to degas molten steel by use of a ladle cover degasser which, in effect, is a variation of the tank degasser systems. The principle difference is that the ladle, having a sealing flange installed, acts as a vacuum tank. In this type degasser, the size of the ladle cover is relatively small. Again, typically an O-ring seal is employed for sealing the flanges on the cover and the ladle. The off gas connection is through the ladle cover. A ladle cover degasser is shown in U.S. Pat. No. 3,201,226.
It is also known to employ an oxygen lance for injecting oxygen onto liquid steel during vacuum treatment in a tank degasser. These systems are commonly referred to as vacuum oxidation decarbonization or VOD systems, and are employed mainly for the reduction of carbon in steels having a high chromium content.
When using a tank degasser as a VOD system for the production of very low carbon stainless steels, a cooled, vertically movable oxygen lance extends through the top of the tank cover for injecting oxygen onto the molten metal during vacuum treatment. Under vacuum, oxygen preferentially combines with carbon over relatively highly oxidizable alloys. The above-mentioned U.S. Pat. No. 3,201,226 also shows a system operating in this manner.
Large quantities of ultra low carbon steel, i.e., steels having carbon content of 50 ppm or lower, are currently used by the sheet metal forming industry, e.g., the automobile and appliance press shops. These steels, especially when microalloyed with titanium and other metals, often called interstitial free or IF steels, have excellent formability even under adverse conditions, and are produced commercially by processes which include vacuum degassing. Recirculating type degassers are widely used for the production of these grades largely because of the high production capacity of such degassers. As is known, however, recirculation type degassing systems are more expensive than tank degassers not only from the standpoint of initial cost, but also from the operating standpoint. Accordingly, while it would be advantageous in some steel plants to employ a tank degasser for the production of ultra low carbon steel, the known tank degassers generally have not been readily adaptable to such high output vacuum degassing operations.
When decarbonizing in vacuum, large volumes of carbon monoxide gas can be formed. The evolving gas volume is further increased by the gas used for stirring and the dissolved gases such as hydrogen and nitrogen which are removed under vacuum. This can produce a violent boiling or stirring action, which is beneficial from a metallurgical standpoint because of the increased surface area of the metal being exposed to the vacuum, enhancing the rate of desired metallurgical reaction. At the same time, the violent splashing will result in formation of buildups and skulls on degasser components which may restrict the flow of off gases to the vacuum pumps and may adversely affect structural integrity and cause thermal distortion, thereby effecting the vacuum seal.
Tank degassers in commercial use generally have a refractory lined or water cool splash shield suspended from the tank cover to prevent the splashed liquid steel and/or slag from reaching the vacuum tank cover. The tank covers, per se, generally are not able to withstand exposure to the extremely high temperatures, particularly those resulting from localized contact with the splashed liquid. Distortion of the tank cover can not only interfere with the seal, but also adversely affect operation of sensitive mechanical equipment required for operation, testing and alloy additions, and the like. The splash shield thus is generally considered essential for protecting the cover, but at the same time effectively reduces the height available for splashing, i.e., the height from which steel or slag particles return to the steel in the ladle. Buildups on the shields inside the tank and offtake not only restrict gas flow but also results in expensive maintenance. To overcome these problems, it is common practice to treat smaller quantities of metal than is normally contained in a ladle. By reducing the volume of liquid steel in the ladle, more freeboard is provided--often 30 to 50 inches--for the desired stirring and splashing to expose the metal to vacuum, but this inherently results in a reduced output or production.
It is, therefore, an object of the present invention to provide an improved tank or ladle cover type vacuum degassing facility capable of vacuum treatment of liquid steel at a high rate.
Another object is to provide an improved method of producing vacuum degassed steel.
Another object of the invention is to provide an improved vacuum degassing apparatus which is efficient in operation and which requires a minimum of maintenance.
Another object is to provide a method of and apparatus for producing vacuum degassed steel in a degasser employing a cover assembly which permits effective degassing at an increased production rate.
Another object is to provide an improved high production tank or ladle cover type vacuum degassing facility which is more economical to build, install, operate and to maintain than typical recirculating degassers.